1. Field of the Invention
The present invention relates to a semiconductor device and more particularly, to a semiconductor device with a silicide contact structure and a fabrication method thereof.
2. Description of the Prior Art
In recent years, the need of making the contact junction shallower to suppress the short channel effects has been increasing more and more with the progressing integration level of electronic elements or components. Thus, it has been an essential problem to be solved to form a contact structure with an improved interface flatness and a more uniform crystal structure.
Also, to form a self-aligned contact structure, a silicide of a metal such as titanium (Ti) and cobalt (Co) has been used. However, in this case, the solid-phase silicidation reaction of these metals does not progress uniformly and as a result, needle-like metal crystals tend to be formed and the crystal structure of a diffusion region such as a source/drain region tends to be damaged. Thus, leakage current tends to flow through the contact structure.
Further, the metal silicide itself is easy to react with aluminum (Al) that is a main constituent of a metallic wiring line to be electrically connected to the contact structure. Especially, if the metal silicide contains a lot of crystal-grain boundaries, a barrier film needs to be additionally formed to withstand a subsequent fabrication process or processes necessitating the process temperature of 600xc2x0 C. or higher.
One reason why the solid-phase silicidation reaction progresses non-uniformly is that various types of silicides having different compositions and crystal structures from one another are generated. For almost all metals that generate silicide, a silicon-poor silicide phase is generated at a comparatively low temperature of silicidation reaction due to comparatively low free energy. On the other hand, at a comparatively high temperature, the silicon-poor silicide phase is turned to a silicon-rich silicide phase with comparatively high heats of formation. Accordingly, a non-uniform silicide film tends to be generated in a heat treatment process of the fabrication process sequence for silicidation reaction dependent upon the crystalline nucleus density of the silicide crystals and the raising rate of the heat-treatment temperature.
As an example of uniform silicide films, an epitaxial cobalt silicide (CoSi2) film is obtained by heat treating a cobalt film on a (111)-oriented surface of a single-crystal silicon (Si) substrate. The epitaxy of the CoSi2 film is due to the fact that Si and CoSi2 have approximately the same lattice constant and that the (111)-interface is energetically favorable. In the epitaxial film, crystal-grain boundaries scarcely exist and consequently, the crystal structure scarcely tends to vary due to heat treatment. This means that a highly reliable contact may be formed.
However, a (100)-oriented surface of a single-crystal silicon substrate has been usually used for semiconductor devices. In this case, a silicide phase such as the CoSi phase having a lower free energy tends to be generated at the start of a solid-phase silicidation reaction. Thus, a uniform epitaxial silicide film is difficult to be formed.
An improvement to solve this problem about non-uniformity is disclosed in an article, Applied Physics Letters, Vol. 58, pp. 1308, 1991. In this improvement, a titanium (Ti) film is additionally provided at the interface between a cobalt film and a single-crystal silicon substrate, thereby forming an epitaxial CoSi2 film on a (100)-oriented surface of the Si substrate. However, in this case, there arises a problem that the epitaxial CoSi2 film does not have a satisfactory thickness uniformity.
Another improvement to solve the above problem about non-uniformity is disclosed in an article, Applied Physics Letters, Vol.68, pp.3461, 1996. In this improvement, a silicon oxide (SiOx) film is additionally provided at the interface between a cobalt film and a single-crystal silicon substrate, thereby forming an epitaxial CoSi2 film on a (100)-oriented surface of the Si substrate. However, in this case, there arises a problem that the epitaxial CoSi2 film does not have a satisfactory large thickness.
As described above, with the conventional formation method of a silicide contact structure where a metal film is directly deposited on a single-crystal silicon substrate and the metal film and the substrate are subjected to a heat treatment, a uniform silicide film is unable to be formed. Thus, it is difficult to form reliably a p-n junction with a small depth of 50 nm or less.
Moreover, because silicides are easy to react with aluminum (Al) that is a main constituent of a metallic wiring line to be electrically connected to the contact structure, a barrier film needs to be formed on the silicide film. This makes it complicated to form the contact structure.
Accordingly, an object of the present invention is to provide a semiconductor device having a reliable contact structure and a fabrication method thereof.
Another object of the present invention is to provide a semiconductor device having a reliable contact structure even if a p-n junction has a small depth of 50 nm or less and a fabrication method thereof.
Still another object of the present invention is to provide a semiconductor device in which a reliable contact structure is formed by simplified processes and a fabrication method thereof.
The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description.
According to a first aspect of the present invention, a semiconductor device is provided, which is comprised of a single-crystal silicon substrate having a main surface, a dielectric film formed on the main surface of the substrate and having a contact hole uncovering the main surface of the substrate, a metal silicide film contacted with the main surface of the substrate in the contact hole of the dielectric film, a silicon nitride film contacted with the metal silicide film in the contact hole of the dielectric film, and an electrically conductive film formed on the dielectric film and electrically connected with the main surface of the substrate through the metal silicide film and the silicon nitride film in the contact hole of the dielectric film.
The metal silicide film has a property that a metal atom of the metal silicide film serves as diffusion species in a solid-phase silicidation reaction for forming the metal silicide film.
With the semiconductor device according to the first aspect of the present invention, the silicon nitride film is formed to be contacted with the metal silicide film in the contact hole of the dielectric film, and the metal silicide film has a property that a metal atom of the metal silicide film serves as diffusion species in a solid-phase silicidation reaction for forming the metal silicide film.
Therefore, if a metal film for the metal silicide film is formed on the silicon nitride film and then, the metal film is subjected to a heat treatment for silicidation reaction, the atoms of the metal film diffuse into the main surface of the substrate through the silicon nitride film during a heat treatment for silicidation reaction, thereby forming the metal silicide film on the main surface of the substrate.
In this case, the atoms of the metal film are able to be suppressed from diffusing at the start of the heat treatment process for silicidation reaction. Thus, a silicide phase with the highest heat of formation is generated after the heat-treatment temperature is completely raised to a specific value. Accordingly, the silicon nitride film is left on the metal silicide film after the solid-phase silicidation reaction.
As a result, the crystal structure at the interface of the metal silicide film and the substrate is satisfactorily uniform and flat, which prevents leakage current from occurring at the interface between the metal silicide film and the substrate. This means that this semiconductor device has a reliable contact structure even if a p-n junction has a small depth of 50 nm or less.
Also, the silicon nitride film thus left serves as a barrier against the electrically conductive film formed on the silicon nitride film and consequently, this semiconductor device is fabricated by a simplified contact formation process.
In a preferred embodiment of the semiconductor device according to the first aspect of the present invention, the metal silicide film is a silicide of a metal selected from the group consisting of cobalt (Co), nickel (Ni), vanadium (V), platinum (Pt), and palladium (Pd). These metals have satisfactorily low heats of formation for nitrides and therefore, they scarcely react with the silicon nitride film. Thus, if the metal atoms diffuse by a diffusion length that corresponds to the film thickness of 10 nm or more, the structure of the silicon nitride film is kept unchanged.
Within these metals, Co is more preferred. In this case, there is an additional advantage that the metal silicide film has a single-crystal structure and a flatter interface.
In another preferred embodiment of the semiconductor device according to the first aspect of the present invention, the silicon nitride film has a thickness of 0.5 to 1.5 nm. If the thickness is less than 0.5 nm, the thickness of the silicon nitride film is difficult to be controlled. If the thickness is greater than 1.5 nm, the diffusion of the metal atoms is excessively slow and therefore, this film is not practically utilized.
A more preferred thickness of the silicon nitride film is 0.5 to 1.0 nm. If the thickness is equal to or less than 1.0 nm, a satisfactory diffusion rate of the metal atoms is obtained.
In still another preferred embodiment of the semiconductor device according to the first aspect of the present invention, the metal silicide film has a single-crystal structure. In this case, there is an additional advantage that the metal silicide film has a flatter interface.
According to a second aspect of the present invention, a fabrication method of a semiconductor device is provided, which is comprised of the following steps (a) to (f).
(a) A single-crystal silicon substrate having a main surface is provided.
(b) A dielectric film having a contact hole uncovering the main surface of the substrate is formed on the main surface of the substrate.
(c) A silicon nitride film is formed on the uncovered main surface of the substrate in the contact hole of the dielectric film.
(d) A metal film is formed on the dielectric film to be contacted with the silicon nitride film in the contact hole of the dielectric film.
The metal film has a property that an atom of the metal film serves as diffusion species in a solid-phase silicidation reaction.
(e) The metal film, the silicon nitride film, the dielectric film, and the substrate are heat-treated to thereby form a metal silicide film due to a solid-phase silicidation reaction between the metal film and the substrate.
The metal silicide film thus formed is contacted with the main surface of the substrate in the contact hole of the dielectric film.
The silicon nitride film is left on the metal silicide film thus formed.
(f) An electrically conductive film is formed on the dielectric film to be contacted with the silicon nitride film.
The electrically conductive film is electrically connected to the substrate through the silicon nitride film and the metal silicide film in the contact hole of the dielectric film.
With the fabrication method of a semiconductor device according to the second aspect of the present invention, after the silicon nitride film is formed on the main surface of the substrate in the contact hole of the dielectric film, the metal film is formed on the dielectric film to be contacted with the silicon nitride film in the contact hole of the dielectric film. The metal film has a property that an atom of the metal film serves as diffusion species in a solid-phase silicidation reaction.
Subsequently, the metal silicide film is formed by a heat treatment due to a solid-phase silicidation reaction between the metal film and substrate. The metal silicide film thus formed is contacted with the main surface of the substrate in the contact hole of the dielectric film. The silicon nitride film is left on the metal silicide film thus formed.
Accordingly, the atoms of the metal film are able to be suppressed from diffusing at the start of the heat treatment step (e) for silicidation reaction. Thus, a silicide phase with the highest free energy is generated after the heat-treatment temperature is completely raised to a specific value. Accordingly, the silicon nitride film is left on the metal silicide film after the solid-phase silicidation reaction.
As a result, a semiconductor device having a reliable contact structure is fabricated even if a p-n junction has a small depth of 50 nm or less.
Also, the silicon nitride film thus left on the metal silicide film serves as a barrier against the electrically conductive film formed on the dielectric film and consequently, this semiconductor device is fabricated by a simplified contact formation process even if a p-n junction has a small depth of 50 nm or less.
A preferred thickness of the silicon nitride film is 0.5 to 1.5 nm. If the thickness is less than 0.5 nm, the thickness of the silicon nitride film is difficult to be controlled. If the thickness is greater than 1.5 nm, the diffusion of the metal atoms is excessively slow and therefore, this film is not practically utilized.
A more preferred thickness of the silicon nitride film is 0.5 to 1.0 nm. If the thickness is equal to or less than 1.0 nm, a satisfactory diffusion rate of the metal atoms is obtained.
As the metal atoms, cobalt (Co), nickel (Ni), vanadium (V), platinum (Pt), and palladium (Pd) are preferably used. These metals have satisfactorily low free energy for nitride and therefore, they scarcely react with silicon nitride. Thus, if the metal atoms diffuse by a diffusion length that corresponds to the film thickness of 10 nm or more, the structure of the silicon nitride film is kept unchanged.
When cobalt (Co) is used as the metal, a CoSi2 film has an epitaxial structure with respect to the silicon substrate. This is because CoSi2 has an approximately the same crystal structure and approximately the same lattice constant.
Also, the diffusion rate of Co within the silicon nitride film is extremely lower than that within a metal. Therefore, the CoSi2 film becomes uniform in crystal structure and poor in defect compared with the conventional device where a Ti film is additionally provided between the substrate and a SiNx film.
In another preferred embodiment of the method according to the second aspect, a step of removing said silicon nitride film is added between the steps (e) and (f).
In this case, there is an additional advantage that the contact resistance is further lowered.